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mm: Add kmap_local_folio()

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This allows us to map a portion of a folio.  Callers can only expect
to access up to the next page boundary.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
This commit is contained in:
Matthew Wilcox (Oracle) 2020-12-30 10:21:39 -05:00
parent 08b0b0059b
commit 53c36de070
2 changed files with 48 additions and 0 deletions
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11
include/linux/highmem-internal.h
View File

@ -73,6 +73,12 @@ static inline void *kmap_local_page(struct page *page)
return __kmap_local_page_prot(page, kmap_prot);
}
static inline void *kmap_local_folio(struct folio *folio, size_t offset)
{
struct page *page = folio_page(folio, offset / PAGE_SIZE);
return __kmap_local_page_prot(page, kmap_prot) + offset % PAGE_SIZE;
}
static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
{
return __kmap_local_page_prot(page, prot);
@ -171,6 +177,11 @@ static inline void *kmap_local_page(struct page *page)
return page_address(page);
}
static inline void *kmap_local_folio(struct folio *folio, size_t offset)
{
return page_address(&folio->page) + offset;
}
static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
{
return kmap_local_page(page);

37
include/linux/highmem.h
View File

@ -96,6 +96,43 @@ static inline void kmap_flush_unused(void);
*/
static inline void *kmap_local_page(struct page *page);
/**
* kmap_local_folio - Map a page in this folio for temporary usage
* @folio: The folio containing the page.
* @offset: The byte offset within the folio which identifies the page.
*
* Requires careful handling when nesting multiple mappings because the map
* management is stack based. The unmap has to be in the reverse order of
* the map operation::
*
* addr1 = kmap_local_folio(folio1, offset1);
* addr2 = kmap_local_folio(folio2, offset2);
* ...
* kunmap_local(addr2);
* kunmap_local(addr1);
*
* Unmapping addr1 before addr2 is invalid and causes malfunction.
*
* Contrary to kmap() mappings the mapping is only valid in the context of
* the caller and cannot be handed to other contexts.
*
* On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
* virtual address of the direct mapping. Only real highmem pages are
* temporarily mapped.
*
* While it is significantly faster than kmap() for the higmem case it
* comes with restrictions about the pointer validity. Only use when really
* necessary.
*
* On HIGHMEM enabled systems mapping a highmem page has the side effect of
* disabling migration in order to keep the virtual address stable across
* preemption. No caller of kmap_local_folio() can rely on this side effect.
*
* Context: Can be invoked from any context.
* Return: The virtual address of @offset.
*/
static inline void *kmap_local_folio(struct folio *folio, size_t offset);
/**
* kmap_atomic - Atomically map a page for temporary usage - Deprecated!
* @page: Pointer to the page to be mapped